A cellulose acetate film is used in various photographic or optical elements because it has tough and enough flame retardant properties. The cellulose acetate film is a representative photogrpahic support. Further, the cellulose acetate film has an optical isotropy. Accordingly, the film is also used in a liquid crystal display device, which has recently extended its market. The cellulose acetate film is used as a protective film of a polarizing plate or a color filter in the liquid crystal display device.
The acetic acid content and the polymerization degree (which has a correlation with the viscosity) of cellulose acetate influence the mechanical strength and the durability of a film obtained from the cellulose acetate. The elasticity, folding endurance, dimensional stability and resistance to moisture and heat decrease with decreasing the acetic acid content and the polymerization degree. An acetic acid content of 58% or more (preferably 59% or more) is necessary to satisfy the required quality of the photogrpahic support or the optical film. The cellulose acetate having an acetic acid content of 58% or more is referred to as triacetyl cellulose (TAC). With respect to the polymerization degree, cellulose acetate preferably has a viscosity average degree of polymerization of not less than 270, and more preferably of not less than 290.
A cellulose acetate film is prepared according to a solvent cast method or a melt cast method. The solvent cast method comprises the steps of casting a solution of cellulose acetate in a solvent (that is called "dope") on a support, and evaporating the solvent to form a film. The melt cast method comprises the steps of casting molten cellulose acetate on a support under heating, and cooling it to form a film. The solvent cast method can form a highly flat film, compared with the melt cast method. Therefore, the solvent cast method is generally employed to give a cellulose acetate film.
The solvent cast method is described in various documents. The recent object of the method is to shorten the period of time between casting the dope on the support and peeling the formed film off the support. If the time is shortened, the productivity of the film formation is improved. For examples, Japanese Patent Publication No. 5(1993)-17844 discloses a process of casting a concentrated dope on a cooled drum to shorten the period of time between the casting step and the peeling step.
The solvent used in the solvent cast method must have functions not only of dissolving the cellulose acetate but also of forming an excellent film. In more detail, the viscosity and the polymer concentration of the solution (dope) should be appropriately adjusted to form a flat plane film having a uniform thickness. The dope also should have enough stability. Further, the dope should easily be set to gel. Furthermore, the formed film should easily be peeled off the support. The most appropriate solvent must be selected to satisfy these requirements. Moreover, the solvent should be so easily evaporated that the solvent scarcely can remain in the film.
Various organic solvents have been proposed as the solvents of cellulose acetate. However, only methylene chloride satisfies all of the above-mentioned requirements. Accordingly, solvents other than methylene chloride have not been practically used.
However, the use of hydrocarbon halides such as methylene chloride has recently been restricted severely to protect the global environmental conditions. Further, methylene chloride is apt to vaporize in the process for the preparation of the film, because it has a low boiling point (41.degree. C.). Accordingly, methylene chloride may cause problems in the working environment. Therefore, the process is conducted under closed conditions. However, there is a technical limitation on sealing methylene chloride in a closed system. Accordingly, it is an urgent necessity to search for a new solvent for the cellulose acetate, namely replacement of methylene chloride.
By the way, acetone is a widely used organic solvent. Acetone has an appropriate boiling point (56.degree. C.). The process of evaporating acetone does not need a large thermal energy. Further, acetone has few problems on the human body and the global environmental conditions, compared with the organic chloride solvents.
However, cellulose acetate has a poor solubility in acetone. Cellulose acetate having a degree of substitution of not more than 2.70 (acetic acid content: 58.8%) is slightly soluble in acetone. The solubility decreases with increasing the substitution degree. Cellulose acetate having the substitution degree of not less than 2.80 (acetic acid content: 60.1%) is not soluble in acetone, and is merely swelled in acetone.
J. M. G. Cowie et al. report in Makromol., Chem., 143 (1971) 105-114, that cellulose acetate having a substitution degree in the range of 2.70 (acetic acid content: 60.1%) to 2.80 (acetic acid content: 61.3%) is dissolved in acetone by a specific process. The process comprises the steps of cooling the cellulose acetate in acetone to a temperature of -80.degree. to -70.degree. C., and warming it to obtain 0.5 to 5 wt. % solution of the cellulose acetate in acetone. The method of cooling the mixture of cellulose acetate in acetone to obtain a solution is hereinafter referred to as a cooling dissolution method. The dilute (0.5 to 5 wt. %) solution reported in J. M. G. Cowie et al. is not appropriate for preparation of a cellulose acetate film. The dope for the preparation of the film requires a cellulose acetate concentration in the range of 10 to 30 wt. %.
The solution of cellulose acetate in acetone is also reported by K. Kamide et al., Textile Machinery Society, Vol. 34, 57-61 (1981). The report (written in Japanese) is entitled "Dry spinning process using acetone solution of triacetyl cellulose." In the report, the cooling dissolution method is applied to the art of fiber spinning. The experiments shown in the report examine the mechanical strength, the dyeing property and the sectioned shape of the fiber obtained by the cooling dissolution method. In the report, 10 to 25 wt. % solution of cellulose acetate is used to form a fiber.